1. Field of the Invention
This invention relates generally to a nulling antenna system and, more particularly, to a nulling direct radiating array antenna that employs auxiliary antenna arrays positioned around a main array for increasing the nulling resolution.
2. Discussion of the Related Art
Various communications systems, such as certain telephone systems, cable television systems, internet systems, and military communications systems, make use of satellites orbiting the Earth to transfer communication signals. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and the satellite retransmits the signal to another satellite or to the Earth as a downlink communications signal to cover a desirable reception area depending on the particular use. The satellite is equipped with an antenna system including an array of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth.
Satellite-based phased array antenna systems have been developed that provide signals to communication areas using pixel beams designed to cover specific areas on the Earth's surface. Typically, the pixel beams are organized into a matrix of evenly shaped and spaced beams to provide a total coverage area for a large geographical area, such as the visible Earth. One particular phased array suitable for this purpose is the "Enhanced Direct Radiating Array" disclosed in U.S. patent application Ser. No. 09/443,526, filed Nov. 19, 1999, assigned to the assignee of this application and herein incorporated by reference.
FIG. 1 is a hexagonal coverage area 10 including cells 12 defined by a phased array antenna system, where each cell 12 represents a pixel beam. The antenna system may provide a plurality of communications channels where each channel includes a plurality of pixel beams. In this example, each channel includes a hexagonal group 14 of seven cells 12, where each cell 12 in each group 14 is labeled A-G. The particular user may be located in the center cell 12 of the group 14, where the perimeter cells 12 in the group 14 provide for increased communications performance. Communications signals from locations in the group 14 are received by the antenna system on the satellite, and then retransmitted to another group 14 for communications purposes. The phased array antenna system provides beam steering for all of the groups 14.
Intentional and unintentional jamming of satellite uplink signals occurs in various situations. For example, in a military situation, satellite communications are used to transmit signals and information to and from a warfare theatre or hostile environment. The reception area for the uplink communications signals in the hostile environment may be jammed by the enemy using a high powered transmitter. If the jamming signal comes from with-in the channel area for the uplink signal, it is referred to as in-beam jamming, and if it comes from outside of the channel area for the uplink signal, it is referred to as out-of-beam jamming. The jamming signal must be at the frequency of the uplink signal to be effective for jamming purposes. Jamming signals can also come from unintentional or friendly sources that inadvertently interfere with the satellite uplink signals.
In order to eliminate or reduce the effects of jamming signals in both hostile and friendly scenarios, it is known to employ nulling antenna systems that detect the presence of a jamming signal, and provide an antenna null in the antenna radiation or reception pattern so that the jamming signal does not significantly affect the uplink signal. Particularly, nulling antenna systems are able to determine the direction of the jamming signal and create a null or void in the radiation pattern of the antenna so that it in effect does not see the jamming signal. In order to be able to block or null the jamming signal so that it does not affect the ability to transmit the downlink signal, it is necessary to determine the location of the signal, whether it be from an in-beam or out-of-beam jamming source, and then provide the null at that location.
An adaptive weighting system is generally used in nulling antenna systems to sample the received pixel beams in a particular channel to determine if a jamming signal is present. The weighting system then weights the pixel beams in the channel to block the jamming signal. The weighting system generally includes a correlator to correlate each of the pixel beams with the combined beam for the channel to determine if a jamming signal is present. Once the correlator determines that a jamming signal is present, algorithms are used to determine the location of the jamming signal. The algorithm goes through each pixel beam separately using a weighting function to determine where the jamming signal is being received from. The weighting function provides the null by inverting the phase of the received signal at the appropriate location. When the weighting of the pixel beams blocks the jamming signal and the image is cleared up, the antenna system knows where the jamming signal is being received from, and can make weighting adjustments accordingly. Various algorithms that perform this function are known to those skilled in the art.
The nulled area of the radiation pattern of the antenna has a width and a depth which determines its effectiveness in nulling the jamming signal. However, creating a null in the radiation pattern of the antenna also creates a "blind spot" in the uplink signal. Therefore, it is desirable to limit the size of the null while still blocking the jamming signal. In other words, it would be desirable to provide higher nulling resolution to tightly define the null in the radiation pattern so that more of the uplink signal can be processed by the antenna system. This would minimize the area of the radiation pattern that is nulled, and still provide effective anti-jamming. In this manner, it is possible to provide communication to a wider area around the jamming source.
It is known by antenna theory to narrow the antenna radiation pattern by increasing the aperture size of the antenna, i.e., providing more antenna elements. However, adding more antenna elements to increase the aperture size significantly increases the cost and complexity of the antenna system. It would be desirable to increase the aperture of the nulling antenna, without significantly increasing the number of elements to provide more effective nulling capabilities. It is therefore an objection of the present invention to provide such a nulling antenna.